The proposed work intends to advance our understanding of cardiac electrogenesis by identifying the explicit role of calcium in this process. Canine Purkinje fibers and rabbit sinus node cells will be studied as representative examples of fast and slow responding cells of the mammalian myocardium. Glass microelectrodes will be used to record membrane potential and to inject polarizing current while exposing the cell under study to various experimental perturbations designed to affect the distribution of calcium across the cell membrane. Through careful evaluation of specific electrical events occurring during the action potential and during electrical diastole, it should be possible to demonstrate the role of calcium in the phenomena of excitability, autorhythmicity and conduction in the heart. It is further hoped that this investigation will lead to the localization and characterizations of receptor mechanisms responsible for mediating calcium actions on monovalent cation permeability.